Base station antenna arrangement

ABSTRACT

An antenna for a base station comprising a plurality of antenna arrays (40; FIG. 6a) each capable of forming a multiplicity of separate overlapping narrow beams in azimuth, the arrays being positioned such that the beams formed by the arrays provide a coverage in azimuth wider than each array. Means are provided for operating (FIG. 12a, FIG. 12b) two or more non-collocated narrow beamwidth antenna arrays to form jointly a broad beamwidth antenna radiation pattern wherein the time averaged antenna pattern is substantially null free.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.08/289,920, filed Aug. 12, 1994.

BACKGROUND OF THE INVENTION

This invention relates to a base station antenna arrangement, for use ina Cellular Radio communications system, which shall hereafter bereferred to as a smart antenna.

TECHNICAL FIELD

Cellular radio systems are currently in widespread use throughout theworld providing telecommunications to mobile users. In order to meet thecapacity demand, within the available frequency band allocation,cellular radio systems divide a geographic area to be covered intocells. At the centre of each cell is a base station, through which themobile stations communicate. The available communication channels aredivided between the cells such that the same group of channels arereused by certain cells. The distance between the reused cells isplanned such that the co-channel interference is maintained at atolerable level.

When a new cellular radio system is initially deployed, operators areoften interested in maximising the uplink (mobile station to basestation) and downlink (base station to mobile station) range. The rangesin many systems are uplink limited due to the relatively low transmittedpower levels of hand portable mobile stations. Any increase in rangemeans that less cells are required to cover a given geographic area,hence reducing the number of base stations and associated infrastructurecosts.

When a cellular radio system is mature the capacity demand can oftenincrease, especially in cities, to a point where more, smaller sizecells are needed in order to meet the required capacity per unit area.The process used to create these smaller cells is known as cellsplitting. Any technique that can provide additional capacity withoutthe need for cell-splitting will again reduce the number of base stationsites and associated infrastructure costs.

The antenna used at the base station site can potentially makesignificant improvements to the range and capacity of a cellular radiosystem. The ideal base station antenna pattern is a beam of narrowangular width as shown in FIG. 1a. The narrow beam is directed at thewanted mobile, is narrow in both the azimuth and elevation planes, andtracks the mobile's movements. When compared to an omni-directionalantenna, such a beam will have the dual benefits of having high gain,leading to increased range in thermal noise limited initial deployments,and rejecting interference from co-channel reuse cells allowing highercapacity without cell splitting in mature deployments. The narrow beamreduces interference in a balanced manner on the uplink and downlink. Onthe uplink the base station receiver is protected from interferencegenerated by mobile station transmitters in the co-channel reuse cells,FIG. 1b. On the downlink the mobile is unlikely to be in the beams ofthe base station transmitters in the co-channel reuse cells. The extentof the advantage of a narrow beam antenna over an omni-directionalantenna is a function of the beamwidth. The narrower the beamwidth thegreater the advantage, but this must be traded off against the increasedsize and complexity of the antenna.

Although the narrow beam is formed at radio frequencies (typically inthe 900 or 1800 MHz bands) it can usefully be visualised as analogous toa laser beam that emanates from the base station and tracks the mobiles.When contrasted with an omni-directional antenna, this clearly creates ahigh quality transmission path with minimal interference. For thepurposes of this document the use of the word "omni" is intended toconvey the meaning of having radiation coverage over the areacorresponding to the required geographic area of the cell.

BACKGROUND ART

Some of the potential benefits of narrow beam antennas, for cellularradio, have been recognised in the literature, see for example "ASpectrum Efficient Cellular Base Station Antenna Architecture", S. C.Swales and M. A. Beach, Personal & Mobile Radio CommunicationsConference, Warwick, 1991 and "Proposed Advanced Base Station Antennasfor Future Cellular Mobile Radio Systems", W. S. Davies, R. J. Long andE. Vinnal, Australian Telecomms Research, Vol. 22, No. 1, pp 53-60.Within current systems the manner in which directive antennae are usedallows relatively small benefits to be obtained. The use of directiveantennas in current cellular radio systems is based on the principle ofsectorisation as illustrated in FIG. 2. The main sources ofinterference, in a cellular system, come from the so called first tierreuse cells. An omni-directional base station antenna will receiveinterference from all six first tier reuse cells, FIG. 2a. If an antennawith nominally 120° beamwidth is used, corresponding to a tri-sectoredconfiguration, interference will be received from only two first tierreuse cells, FIG. 2b. If an antenna with 60° beamwidth is used,corresponding to a hex-sectored configuration, interference will bereceived from only one of the first tier cells, FIG. 2c. In sectorisedcells the cellular radio transceivers at the base station are onlyconnected to one sector (or antenna) and cannot be used in other sectorswithin the same cell.

The sectorised approach to the use of directive antennas has reached itsuseful limit at 60° beamwidth and can go no further. There are two keydisadvantages of the approach:

a) The cellular radio transceivers are dedicated to particular sectorsthat leads to significant levels of trunking inefficiency. In practicethis means that many more transceivers are needed at the base stationsite than for an omni-directional cell of the same capacity.

b) Each sector is treated by the cellular radio network (i.e. the basestation controller and mobile switches) as a separate cell. This meansthat as the mobile moves between sectors, a considerable interaction isrequired, between the base station and the network, to hand off thecall, between sectors of the same base station. This interaction,comprising signalling and processing at the base station controller andswitch, represents a high overhead on the network and reduces capacity.

A standard cellular radio system is comprised of several layers, asshown in FIG. 3. A Mobile Switching Centre (MSC) is the interfacebetween the cellular system and other networks, e.g. PSTN, PublicSwitched Telephone Network or ISDN, Integrated Services Digital Network.Each MSC controls several Base Station Systems (BSS), which in somesystems, such as GSM or PCS, are further divided into a Base StationController (BSC) which controls several Base Transceiver Stations (BTS).Each BSS communicates with several Mobile Stations (MS). At the MSClevel there are also other facilities such as Operations and Maintenance(OMC) and Network Management (NMC).

In this system the calls are allocated to transceivers at baseband inthe cellular radio network, at either the BSC, if available, or at theMSC, as shown in FIG. 4a. Any change required in the call to transceiverallocation has to be signalled through the network, maybe as far as theMSC and back again. This represents a heavy loading on the signallingnetwork and a time delay whilst it occurs.

The basic concept of a smart antenna is disclosed in European PatentApplication No. 92 309 520.2. A smart antenna as referred to hereinaftercomprises a plurality of antenna arrays each capable of forming amultiplicity of separate overlapping narrow beams in azimuth, the arraysbeing positioned such that the totality of beams formed by the arraysprovides a substantially omni-directional coverage in azimuth, azimuthand elevation beamforming means for each array, a plurality of r.f.transceivers each for transmitting and receiving r.f. signals for one ormore calls, switching matrix means for connecting each transceiver withone or other of the arrays via the beamforming means, control means forcontrolling the switch matrix means whereby a particular transceiver isconnected to a particular array, via the beamforming means, to exchanger.f. signals with a remote station located in the area covered by one ofthe narrow beams.

SUMMARY OF THE INVENTION

According to the present invention there is provided an antennacomprising:

a plurality of layered antenna arrays each capable of forming amultiplicity of separate overlapping narrow beams in azimuth, the arraysbeing positioned such that the beams provide a coverage in azimuth widerthan each array; characterised in that means are provided for operatingtwo or more non-collocated narrow beamwidth antenna arrays to formjointly a broad beamwidth antenna radiation pattern wherein the timeaveraged antenna pattern is substantially null free.

Preferably separate phase adjusting means are provided for adjacentantenna arrays. Preferably each of the phase adjusting means can beadjusted independently.

According to a further aspect of the invention, the antenna arrangementincludes;

azimuth beamforming means for each array;

a plurality of r.f. transceivers each for transmitting and receivingr.f. signals for one or more calls;

switching matrix means for connecting each transceiver with one or otherof the arrays via the beamforming means; and

control means for controlling the switch matrix means whereby aparticular transceiver is connected to a particular array, via thebeamforming means, to exchange r.f. signals with a remote stationlocated in the area covered by the antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIGS. 1a and 1b illustrate schematically the use of a narrow beamantenna to communicate between a base station and a mobile station,

FIGS. 2a-2c illustrate schematically the principle of sectorisation of abase station,

FIG. 3 is a block diagram of the main elements of a cellular system,

FIGS. 4a and 4b illustrate the differences in call handling between aconventional cellular system and one using a smart antenna,

FIG. 5 is a block diagram of the main elements of a base station,

FIGS. 6a and 6b are diagrams of the constituents of a multiple narrowbeam base station,

FIG. 7 illustrates the basic principle of a switching matrix,

FIG. 8 illustrates schematically the use of an interference detector,

FIG. 9 illustrates schematically the use of assisted handovermanagement,

FIG. 10 is a block diagram of the communication link between the smartantenna and the rest of a cellular system,

FIG. 11 illustrates pictorially the interfacet radiation pattern of amultifaceted system with and without the use of phase hopping,

FIGS. 12a-12c are diagrams of different embodiments of phase hopping,

FIGS. 13a and 13b illustrate schematically the principles of angulardiversity,

FIGS. 14a-14c are diagrams of different embodiments of the dual transmitbeam system with an illustration of the relative radiation patternimprovements to be found,

FIGS. 15a-15c illustrate the operation of a multiple narrow beam basestation,

FIGS. 16a and 16b illustrate schematically the reduced overlap atdiffering cell radii boundaries using cell dimensioning,

FIGS. 17a and 17b illustrate schematically the flexibility in basestation location by the use of cell dimensioning,

FIG. 18 illustrates schematically the use of cell dimensioning to reduceinterference problems, and

FIG. 19 illustrates schematically the use of cell dimensioning to avoidcongestion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The main elements of a smart antenna as shown in FIG. 5 comprise a mast,tower or building 10 supporting the antenna array(s) 12 and associatedantenna electronics unit 14, which includes beamformers, diplexers andamplifiers. The antenna electronics unit 14 is connected via a cabinelectronics unit 16 to the base station 18 that is under the control ofa base station controller 20. The smart antenna system replaces theconventional passive antenna normally attached to the base station. Theuse of electronics in the masthead allows the call switching to becarried out between the transceivers and antennas within the smartantenna, as shown in FIG. 4b. The switching now occurs on the r.f.signals and only requires local control from the attached base station.This requires a new interface link 17 to be established between the basestation and the smart antenna system. The previous baseband informationis no longer required, reducing the loading on the signalling throughthe cellular radio network. It is replaced by r.f. assignmentinformation on the new interface link between the base station and smartantenna. This interface is also used to convey control information fromthe MSC, OMC and NMC parts of the cellular system.

For the purposes of this description the term "base station network" isused to describe all parts of the cellular system prior to the smartantenna and its interface link, e.g. the radio, base station controller,mobile switching centre, operations and maintenance and networkmanagement.

The detailed constituents of the smart antenna are shown in FIG. 6. Themasthead antenna electronics is shown in FIG. 6a and the cabinelectronics in FIG. 6b. Only one of the antenna arrays is depicted. Eachantenna array 40 comprises a conventional array of individual antennaelements 42 arranged in rows and columns. Each column of elements isenergised via an elevation beamforming network 44. Each elevationbeamforming network combines the elements of a column to a single feedpoint. The amplitude and phase relationships of the r.f. signals coupledto the elevation beamformer determine the elevation beam pattern of theantenna for both transmit and receive. Each elevation beamformer iscoupled to the azimuth beamformer 46. The azimuth beamformer hasmultiple ports for both transmit and receive, one for each elevationbeamformer. The phase and amplitude relationship of the r.f. signalscoupled to the elevation beamformers control the azimuth beam patternfor both transmit and receive. As the azimuth beamformer is prior to thelow noise amplifiers on the receive path it must be optimised for lowloss in that path. One well-known type of beamformer is the Butlermatrix.

The transmit and receive signals for the azimuth beamformer are coupledto the beamformer via individual diplexers 48. Filters that cover justthe transmit or receive frequency bands respectively can be used forthis purpose. In the transmit path the diplexers 48 are fed via acombiner 50 from separate single carrier power amplifiers 52. Theseamplify the r.f. signals up to the power levels required fortransmission. In the receive path the diplexers 48 feed separatesubstantially identical low noise amplifiers 62, one for each azimuthbeam. The low noise amplifiers are required to amplify the weak receivedr.f. signals prior to any system losses to establish a low noise figure(high sensitivity) in the subsequent receive path.

In the receive path, signals are passed from the low noise amplifiers 62to the receive splitter 74. On the transmit side, signals are passed tothe single carrier transmit amplifiers from cell shaping attenuators 54.There is one cell shaping attenuator per transmit amplifier. Allattenuators in any one beam are set to the same value to give a new beamtemplate across all frequencies. This sets the maximum range in aparticular direction, however the power required to reach a particularmobile in the beam can be reduced from this if necessary. Theattenuators are controlled by the operator via the masthead controlelectronics. The cell shaping attenuators are situated prior to theamplifiers, to enable low power standard attenuators to be used. Byplacing them prior to the combiner the intermodulation performance isimproved, due to each being at a single frequency.

Signals are passed from the transceivers 84 to the cell shapingattenuators, by a switching system, via an optional phase hopping module66. This ensures that all transmitters can be connected to anybeamformer input, however only one transmitter is connected to any oneof the single carrier power amplifiers, at any time. The switchingsystem comprises several levels of switching or splitting, which ensuresprimarily maximum redundancy on the omni path and secondarily someredundancy in the traffic paths. The transceivers 84, if required can beinput to an n*n transmit switch matrix 78, where n is equal to thenumber of transceivers. The transmit switch matrix allows any one inputto be connected to any one output, but not more than one input to anyone output simultaneously. This allows for redundancy should any cablein the mast fail, however the same function can be accomplished by theBTS if a suitable command interface exists. A combination of switchesand splitters 56, 58, 68 is used to ensure that the omni path is routedto every beam, whilst a single traffic channel only goes to one beam.This switching and splitting function may be placed either at the top orthe bottom of the mast or a combination of both as shown in FIG. 6. Thepreferred method is to have the main facet switches 68 at the bottom ofthe mast and then each transceiver path is split to every beam, via thebeam splitter 58, where the amplifier select switch matrix 56 switchesoff the beams not required. This makes the implementation of the dualtransmit beam concept far easier and ensures that the lower reliabilitycomponents are in the cabin where access is easier.

The transmit, receive and amplifier select switch matrices comprise anr.f. cross-bar switch that allows any of its inputs to be connected toany of it's outputs. The switch matrix design is such that any number oftransmitters or receivers can be connected simultaneously to any onebeamformer port, thus, if necessary, all the transmitters can beconnected to one beam port at a given time. Likewise all the receiverscan be connected, if necessary, to the same beam port at the same time.In practice, should there be more transceivers than a single beam canhandle, the number of transmitters that can be connected to the beamport is limited by the number of Tx power amplifiers 52. The switchmatrices are operated under the control of a control processor 80. Atypical switch matrix structure is illustrated in FIG. 7.

The receive splitter 74 ensures that all incoming signals, from eachbeam, are sent to the interference discriminator 70; the parallelreceivers 72 and both the main and diverse receive switch matrices 82.

The interference discriminator 70 is used to identify whether or not theincoming signal is from a mobile in its own cell, or one of a nearbycell or any other spurious source. The parallel receivers only assesssignal strength, however, one of the strongest signals may not be from amobile within the cell, as shown by the direct path signal from MS2 inFIG. 8. If these errant signals are not identified, it can lead toerrors in the processing within the base station. All transmissionsbetween a mobile and a base station contain a fixed pattern known as atraining sequence, every base station within a given area has its ownunique training sequence. The interference discriminator selects one ofthe beams, in each timeslot, and searches for the training sequencewithin the received signal, usually using correlation techniques fordigital signals. The beam that is selected is dictated by the controlprocessor, based on information received from the receive switchmatrices and the interference discriminator. It does not necessarilylook at every beam, only those considered to be the most likelycontenders. The use of an interference discriminator is one of thefeatures of the smart antenna system which allows the frequency re-usenumber to be decreased.

A bank of parallel receivers 72, one for each beam, allow every receivechannel to be monitored on every beam simultaneously. For each channelthe receivers measure the quality of the wanted mobile signal present oneach beam. The information on which is the `best` beam is passed to thecontrol processor. The quality measure used by the receivers will varydepending on the particular cellular system concerned. In simple casesthe measure will be the highest power level in other cases carrier tointerference ratio will be used.

The basic function of the control processor 80 is to control thetransmit and receive switch matrices such that the best beam (normallythe one pointing at the mobile stations geographic position) for a givenchannel is selected. The inputs to the control processor are the beamamplitude data from the parallel receivers and data from the controlbuses to the base station. The latter allow the control processor tomonitor a given mobile station's assignment to various control andtraffic channels in the system during the progress of a call. Knowledgeof which channel the mobile is being moved to allows a prompt andnon-disruptive assignment to the best beam. The control algorithms usedwill fall into two basic classes, one for initial acquisition of thebest beam for a new call and one for tracking of the best beam when acall is in progress. It is anticipated that due to different multipathconditions the parameters within the control algorithms will vary forrural and urban cells. The determination of beam selection on the uplinkis used to select the corresponding beam for the downlink. Theinformation on a mobile's angular position, i.e. the present beam beingused, together with real time tracking data from the tracking algorithm,involving range and angular velocity, is sent back, on the transceivercontrol bus via the BTS, to the BSC or MSC as required.

This information can then be directed to the next cell into which themobile will pass. The choice of this next cell is decided based uponpolling of the surrounding cells, either by the mobile or by the basestation controller. If it is by the base station controller, then theinformation from the smart antenna can be used to prioritise the pollingsequence. This will enable the controller to reach the correct decisionquicker, thus reducing the loading on the base station controller.Having chosen the correct cell, with a conventional omni receiver thereis no advantage to knowing the approximate azimuth position of a mobilewithin that cell, however in a multiple beam antenna each beam must bemonitored to find the one containing the mobile. It is therefore a greatadvantage to know the approximate beam into which a mobile will appear,so that the order in which the beams are analysed can be weighted togive priority to the known direction. FIG. 9 shows a mobile passingthrough cell 1 and into cell 2. The tracking algorithm of the smartantenna in cell 1 monitors the mobile's progress through beams 12, 11,10 and 9 and can then give a quite accurate prediction to cell 2 thatthe mobile will appear in one of beams 18, 19 or 20.

The main and diverse receive switch matrices, operate under the controlof the control processor, on information derived from the parallelreceivers, and select the strongest and second strongest signals,respectively. These signals are then coupled by r.f. bus paths to themain and diverse ports of the bank of transceivers 84, one for eachchannel to be provided by the base station, where they are input to amaximal ratio combiner, of the type described in Mobile CommunicationsSystems by J D Parsons et al, Blackie 1989. The transceivers areoperated under the control of the base station controller 88, which alsoprovides overall control for the switch matrix control processor 80.

The transceiver control bus 86 provides the communication link betweenthe base station and the smart antenna. The communication link will becomprised of several buses, whose format will vary according to the typeof base station to which the smart antenna is attached. Whereverpossible the bus structure in the smart antenna will utilise the busprotocol of the base station. In the current implementation there arefive bus types that carry the information outlined below:

1. Operations and maintenance that carries configuration, supervisionand alarm management information for general operation purposes.

2. Operator controlled configuration information originating from eitherthe BSC or the MSC.

3. Frequency values, timing information to identify position within theGSM frame structure, control information, beam power levels and mobilerange. This is from the BTS to the smart antenna, with one bus pertransceiver.

4. Information about the mobile, e.g. signal strength, direction, beamnumber. This is from the smart antenna to the BTS.

5. Signal strobes.

The actual physical link used for communication between the smartantenna and the BSC and/or MSC will preferably be the existingsignalling link, however a separate link as shown in FIG. 10 may also beused.

The key features of the invention can now be considered in more detailand contrasted with the conventional sectorised base station. It is nota single feature of the invention but rather the overall architecture(the functions and their precise disposition) which provides a practicaland economic realisation of the narrow beam concept.

Considered from the network viewpoint, the smart antenna appears as anomni-directional cell site. Since any transceiver can be switched to anybeam and hence look in any direction, there are no sectors. Thus, withinthe network all signalling and processing associated with sector tosector hand-offs are eliminated. Also the fact that transceivers can beused in any direction eliminates the trunking inefficiency of sectorisedsites. These factors not only eliminate a significant load from thenetwork but allow the antenna system to utilise effectively narrowerbeamwidths than would otherwise be possible.

An omni pattern is still necessary as a cellular radio base station isrequired to radiate the BCCh channel over its total arc of coverage, atmaximum power, in all time slots. It may also be required to radiateother carriers at times with the full arc of coverage. In conventionalbase station configurations this is achieved by the use of a single omnior a tri-sectored antenna system with all carriers having the samecoverage pattern. For a smart antenna arrangement, however, a differentsituation exists, in that traffic channels are radiated using selectednarrow beams whilst the base station appears omni-directional to thecellular system. In order to achieve this the antenna arrangement mustgenerate both directional and omni-directional patterns simultaneously.The smart antenna is made of a number of facets each covering a givensector, such that the total coverage is 360°. Each sector contains anumber of beams which generate a sectoral pattern. This can be achievedusing a beam set generated using, for example, a Butler Matrix. Such aset of beams when simultaneously excited by an r.f. carrier will producea sectoral pattern with minimal ripple.

To produce an omni-directional pattern with minimum ripple it isnecessary to place each facet such that their phase centres arecoincident. This is clearly not possible. Practicality dictates that aminimum phase centre spacing of typically 5 or more wavelengths spacingis required.

When the phase centres are separated nulls are produced in the patternwhose position and depths are dependant on the phase centre separation,the rate of cut off of the individual sectoral patterns, electricalphasing errors and mechanical positioning tolerances. A pictorialrepresentation of a typical null pattern at the interfacet region for afour facet mounting is shown in FIG. 11. For the 5 wavelength spacingrequired in a realisable smart antenna this first null assuming perfectphasing and mechanical alignment will occur some 5.6 degrees from thefacet intersect and have a depth of some 6-7 dB that is more than can betolerated. In many installations, e.g. those round buildings spacing of100 or more wavelengths may be required leading to very deep nullsindeed.

This effect does not permit good omni-directional coverage to beobtained. Two possible solutions present themselves. First the cellcould be sectored removing the requirement for omni-directionalcoverage. This could be on the basis of either quad sectoring using fourBCCh channels or bi-sectored using two BCCh channels pointing inopposite directions. The second solution, that is to be discussed indetail, is known as phase hopping, it is this solution that is proposedfor the generation of the omni directional pattern that will then havean effective (time averaged) amplitude ripple of some 2 dB. FIG. 11 alsoshows, overlaid on the null pattern, a thick dashed line illustratingthe improvement obtained when using phase hopping.

With phase hopping the array of facets are fed as two or more groupswith no two adjacent facets being fed from the same output of the phasehopping module. An example of such a phase hopping module is shown inFIG. 12a. In this case the array consists of four facets each covering a90 degree sector. The diametrically opposite facets are connected to thesame feed via a power splitter. Each of these pairs of feeds is moved inphase relative to each other, by the use of a phase shifter. This can beachieved by a single 360 degree phase shifter in one arm. Alternativelytwo lesser value phase shifters capable of providing the full 360 degreerelative phase shift placed one in each arm could be used. This lattersystem in practice will give a better amplitude balance.

The phase shifters are controlled in such a way as to vary the relativephase of the facets through 360 degrees, in a suitable time scale forthe system in question to integrate the received energy and maintain abest average link. In the case of the GSM/DCS1800 type wave forms thisis perhaps best achieved by stepping the phase on a time slot by timeslot basis using say 16 steps controlled in a pseudo random manner. Theuse of a stepped wave form in this case prevents degradation of thephase trajectory response that would occur for a linear phase shift. Therandomisation of the phasing is to prevent any cyclic interference withthe various GSM message formats that occur on a multi-frame basis.

The effective loss of such a system is a maximum of approximately 2 dBrelative to the optimally combined signals when both signals have equalamplitude, being a lesser value for unequal amplitude signals. With sucha system, if the crossover level between facets is at -4 dB, then avirtually uniform averaged omni pattern will result.

An alternative method that will be viable in certain situations is shownin FIG. 12b. This method involves phase hopping between adjacent facetson one of the diagonals, e.g. the interfacet region between 1 & 2 andbetween 3 & 0, but not between 0 & 1 or 2 & 3. This can only be achievedif the facet phase centres of the latter two pairs (0 & 1 and 2 & 3) aresufficiently close together to enable a good beam pattern to be obtainedat this non-cycled interfacet region.

A method that will in effect phase cycle the interfacet region is shownin FIG. 12c. This method involves the use of transmit diversity, themain and diverse ports of the transmitters being connected to adjacentfacets. The diverse port contains the same signal as the main port butdelayed in time. With GMSK modulation, as used in GSM and DCS1800,alternate bits will have an offset from the previous bit equal to amultiple of 90°. This will provide the facets with random 90° phasechanges, a form of phase hopping.

The position of the amplifiers 50, 52 at the top of the mast or buildingis the key to the whole architecture. Firstly the concept of switchingthe transmitters to any beam is impractical unless it can be achievedwithout generating intermodulation products, or at least maintainingthem at a very low level. This is not possible if one were to attempt toswitch the power levels, which can be as high as 50 watts, at thetransceiver outputs. It is necessary to switch before poweramplification. Secondly if power amplification takes place at the footof the mast or building, the r.f. feeder cables must be very low lossand become large and expensive. This would be a significant practicallimitation on the number of beams one could have in a system.

By situating the amplifiers at the top of the mast or building the aboveproblems are solved. However, the precise position in the architecturewithin the antenna electronics unit is still critical. Also since theamplifiers are at the top of the mast they must be extremely reliableand failures should not produce catastrophic degradation in systemperformance.

The positioning of the single carrier power amplifiers 52 prior to thediplexers 48 that are prior to the azimuth beamformer provides anexcellent compromise between the above factors and cost. If a completesingle carrier power amplifier was to fail (which is unlikely because oftheir simple hybrid design that leads to high reliability) the maineffect would be a reduction in traffic capacity in only one beam. Theomni pattern would remain unaffected as this takes precedence inamplifier allocation via the switch matrices. The use of single carrieramplifiers reduces the problems with intermodulation products.Positioning the diplexers prior to the azimuth beamformer requires fewerdiplexers that proves to be a more cost effective solution. It alsosimplifies the control of the amplitude ripple across the beams requiredfor the omni pattern.

A potential disadvantage of the invention is that a relatively largeantenna aperture, in terms of wavelengths, is needed to produce thenarrow beams. If the antenna aperture were very large this could createaesthetic and structural problems, due to wind loading, etc., in somesites. This potential disadvantage is overcome by using the same antennaarray 40 for transmit and receive. In this way the outline of theantenna, for reasonable beamwidth, is less than that of manyconventional cell sites. Thus the diplexing of transmit and receivethrough the common aperture is a key feature of the architecture.

This type of antenna also has the potential disadvantage that it doesnot readily lend itself to the doubling of antenna elements required togain spatial diversity in the receive path. Spatial diversity is themost common method currently in use to overcome the problems ofmultipath. The smart antenna can overcome this problem by using angulardiversity, due to the fact that it has a separate incoming signal ineach one of its multiplicity of beams. These can be compared with thetwo largest signals selected, in the receive switch matrices, and hencediversity can be maintained.

The advantages gained from this invention are twofold, depending onwhether the cellular base transceiver station is sited in an area ofhigh or low multipath. In a high multipath environment the antennareceives strong scattered signals from widely separated angles as shownin FIG. 13a. The antenna will select the two strongest signals,regardless of in which beam they appear, these could for example bebeams B8 and B17 or any combination of the beams shown. These twosignals can then be routed to the main and diverse ports of any maximalratio combiner to maximise receive power and give diversity gain to thesystem.

In a low multipath environment, where strong scattered signals are notas common, the two strongest signals will normally be in adjacent beams,as shown for the mobile station in FIG. 13b. The two strongest signalswill again provide the inputs to a maximal ratio combiner as for thehigh multipath case. Due to the fact that the beams are orthogonal theresultant signal will in effect "fill in" most of the cusp between thetwo beams. Hence in a low multipath environment the ripple in the omnipattern will be reduced from approximately 3.9 dB to the order of 0.9dB, as shown by the shaded area in FIG. 13b, resulting in a possible 3dB gain in received signal power.

Considering the improved coverage pattern shown in FIG. 13b, this canalso be achieved on the transmit side by the use of dual transmit beams.When a mobile passes through the cusp, as shown in FIG. 13b, theswitching system will ensure that the signal is fed to the two adjacentbeams. Dual transmission will only occur if the smart antenna predictsthe mobile to be passing through the cusp or the received signal fromthe mobile is very weak. The use of beam splitters to feed the samesignal to all beams reduces the complexity of the implementation. Thetwo beams can be fed either in phase or in quadrature depending upon thecircumstances. The two beams can be fed at full power in phase, as shownin FIG. 14a, if there is no ERP limitation or if the smart antenna isoperating at least 3 dB below the limit. This will result in a 3 dBimprovement in peak signal power with the 3 dB cusp point at theprevious peak level, as shown by the hatched area in FIG. 14c. If thesystem is operating close to the ERP limit then this method can only beused if the power level fed to the two beams is reduced and will noteliminate cusping completely. A preferred solution in this instance isto feed the two beams in quadrature, as shown in FIG. 14b. This willhave the effect of filling the cusp, as shown by the solid area in FIG.14c, whilst not increasing the peak radiated power level.

FIG. 15 illustrates the system operation. FIG. 15a shows the concept ofa multiplicity of narrow, overlapping beams covering the cell areasurrounding the base station. The beams are referenced b1-b20. FIG. 15bshows how, at time t₁ four mobile stations ms1-ms4 are served by beamsb2, b8 and b17. Beam b2 serves two mobile stations ms2 and ms3 at thistime. As the mobile stations move geographically in relation to the basestation, at time t₂ beam b18 now serves mobile station ms1, b4 servesms3 and b7 serves ms4. Mobile station ms2 has at time t₂ moved out ofthe cell coverage of this base station and will now be served by anadjoining base station (not shown).

The use of cell shaping attenuators enables the contour of the idealcell illustrated in FIG. 15a to be altered. This feature has severaladvantages for the cell planner and the operator; such as the reductionof handovers and lower interference levels, by removing large areas ofoverlap; flexibility of base station location; avoidance of interferencesources and congestion management, each of which will be describedindividually.

Cell planners usually use a hexagonal grid to obtain best coverage andinterference reduction. In rural areas cell size will be limited by thetransmit power of the mobiles and base stations, however in urban areas,cells are also likely to be limited by co-channel interference. Cellplanners therefore have to be able to match together cells of differentsizes at cell split boundaries. FIG. 16a illustrates a typical celllayout, using three cell sizes and the shaded areas indicate the regionsof overlap. It is obvious that there are quite large areas of overlap,along the boundaries between regions of different cell sizes. Areas ofoverlap can also exist between cells of the same size due to shadowingeffects and the canyon effect of streets in large cities, etc.

Large areas of overlap can cause problems with interference and with amuch higher rate of handover, for mobiles, between cells, which can leadto a heavy loading on the network. FIG. 16b shows that by adjusting thepower levels in each of the beams for cells A4, B9 and B11 the areas ofoverlap can be greatly reduced. This will reduce the conflict aboutwhich base station is handling a mobile and the interference caused byoverlapping coverage areas. It will also result in a reduction in powerconsumption for individual cells.

With conventional base station antennas, once the cell grid has beendecided, the operator has little flexibility in where the base stationscan be sited. FIG. 17a shows that to get a coverage of an approximateradius R a conventional BTS must be sited close to the centre of thecell. The present invention, however, due to its cell dimensioningcapability allows the operator a lot more flexibility in the siting ofthe base station, as can be seen in FIG. 17b. This can result in afinancial saving to the operator, by allowing the choice of cheapersites.

This invention also has the means to manage the coverage area aroundpotential sources of interference (or where the BTS would be theinterferer) with the minimum loss of mobile coverage. By attenuatingjust one, or several adjacent beams, it is possible to put a notch inthe antenna footprint, effectively acting as an interference cancellorin a particular direction, as shown in FIG. 18.

Cell Dimensioning can also be used to dynamically control periodiccongestion in cells. Considering FIG. 19, if cell A always experiencesmuch higher traffic density during the morning period, for example, thencells B and C can be increased to reduce the size of cell A (the shadedarea), hence relieving some of the congestion. Later in the day thecongestion may then appear in cell C and cells A and B can be increasedto lower the traffic density in cell C. Periodic adjustments in the cellboundaries can be achieved wherever the traffic density fluctuates in aknown manner during a fixed period of time. Cell sizes can only beincreased in the limit of the transmit power of the antenna, or animposed ERP limit.

This feature can also be used to enable maintenance work to be carriedout when the cell utilisation is low. One cell can be switched off andits neighbouring cells will increase their size to cover the cell,without any loss of coverage.

I claim:
 1. An antenna for a cellular radio base station, the antenna comprising:a plurality of layered antenna arrays each capable of forming a multiplicity of separate overlapping narrow beams in azimuth, the arrays being formed of a plurality of vertically arranged linear antenna arrays placed in a parallel side by side relationship to form a two dimensional array, the two dimensional arrays being positioned such that the beams provide a coverage in azimuth wider than that provided by each vertical array, azimuth beamforming means for each array; a plurality of r.f. transceivers each for transmitting and receiving r.f. signals for one or more calls; switching matrix means for connecting each transceiver with one or other of the arrays via the beamforming means; and control means for controlling the switch matrix means whereby a particular transceiver is connected to a particular array, via the beamforming means, to exchange r.f. signals with a remote station located in the area covered by the antenna, wherein means are provided for operating two or more non-collocated narrow beamwidth antenna arrays to form jointly a broad beamwidth antenna radiation pattern wherein the time averaged antenna pattern is substantially null free.
 2. An antenna arrangement according to claim 1 wherein phase adjusting means are provided for adjacent antenna arrays.
 3. An antenna arrangement according to claim 1 wherein phase adjusting means are provided for adjacent antenna arrays, which phase adjusting means can be adjusted independently.
 4. A method of operating an antenna for a cellular radio base station, the antenna comprising: a plurality of layered antenna arrays each capable of forming a multiplicity of separate overlapping narrow beams in azimuth, the arrays being formed of a plurality of vertically arranged linear antenna arrays placed in a parallel side by side relationship to form a two dimensional array, the two dimension arrays being positioned such that the beams provide a coverage in azimuth wider than that provided by each vertical array, azimuth beamforming means for each array; a plurality of r.f. transceivers; and switching matrix means; the method comprising the steps of:generating and receiving r.f. signals for one or more calls via the plurality of r.f. transceivers; connecting each transceiver with one or other of the arrays via the switching means and beamforming means; and controlling the switching matrix means and beamforming means whereby a particular transceiver is connected to two or more antenna arrays to exchange r.f. signals with a remote station located in the area covered by the antenna, wherein a broad beamwidth antenna radiation pattern is formed, the time averaged antenna pattern of which is substantially null free. 